Quantum key distribution apparatus &amp; method

ABSTRACT

A quantum key distribution (QKD) system is provided that makes use of a quantum signal of polarized photons and comprises a QKD device and complimentary QKD apparatus. The QKD device has a QKD subsystem comprising one of a QKD transmitter and receiver for inter-working with a complimentary QKD receiver or transmitter of said apparatus. The device also has an alignment subsystem arranged to wirelessly interact with the QKD apparatus to enable generation of user feedback and/or adjustment signals for use in aligning the QKD transmitter and receiver such that the QKD transmitter is pointing at the QKD receiver and the polarization axes of the QKD transmitter and receiver are aligned.

FIELD OF THE INVENTION

The present invention relates to quantum key distribution apparatus andmethods.

BACKGROUND TO THE INVENTION

With advances in computing, and in particular with the possibility ofquantum computing platforms becoming available, the once secure publickey infrastructure based on RSA encryption is coming under question. Aspart of the desire to address possible security shortcomings work iscurrently underway to develop a quantum key based infrastructure.

Preferred embodiments of the present invention aim to provide apparatususable in such an infrastructure.

SUMMARY OF THE INVENTION

According to one aspect of the present invention, there is provided adevice for quantum key distribution, herein QKD, using a quantum signalof polarized photons, the QKD device being intended for use withcomplimentary QKD apparatus, the device comprising:

-   -   a QKD subsystem comprising one of a QKD transmitter and receiver        for inter-working with a complimentary QKD receiver or        transmitter of said apparatus; and    -   an alignment subsystem arranged to wirelessly interact with said        apparatus to enable generation of adjustment signals for use in        aligning the QKD transmitter and receiver such that the QKD        transmitter is pointing at the QKD receiver and the polarization        axes of the QKD transmitter and receiver are aligned.

According to another aspect of the present invention, there is providedapparatus for quantum key distribution, herein QKD, using a quantumsignal of polarized photons, the QKD apparatus being intended for usewith complimentary QKD device, the apparatus comprising:

-   -   a QKD subsystem comprising one of a QKD transmitter and receiver        for inter-working with a complimentary QKD receiver or        transmitter of said device; and    -   an alignment subsystem arranged to wirelessly interact with said        device to generate adjustment signals for use in aligning the        QKD transmitter and receiver such that the QKD transmitter is        pointing at the QKD receiver and the polarization axes of the        QKD transmitter and receiver are aligned.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described, by way of example only,with reference to the accompanying diagrammatic drawings of exampleembodiments, in which:

FIG. 1 is a schematic illustration of a quantum key distribution systemembodying the present invention;

FIG. 2 is a schematic illustration of an embodiment of a quantum keydistribution transmitting apparatus usable in the FIG. 1 system;

FIG. 3 is a schematic plan view of an array of light emitting diodesused in the transmitting apparatus of FIG. 2;

FIG. 4 is a schematic illustration of a shaped shutter used to generatea shaped light beam in the transmitting apparatus of FIG. 2;

FIG. 5 is a schematic illustration of an embodiment of a quantum keydistribution receiving apparatus usable in the FIG. 1 system;

FIGS. 6A and 6B together form a functional flow diagram illustrating anexample method of operation of the system shown in FIGS. 1-5;

FIGS. 7A and 7B is a schematic illustration of an arrangement foraligning the optical axes of the transmitting apparatus of FIG. 2 and ofthe receiving apparatus of FIG. 5; and

FIG. 8 is a schematic illustration of a further embodiment of a quantumkey distribution receiving apparatus usable in the FIG. 1 system.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1 of the accompanying drawings there is shown anquantum key distribution (QKD) system comprising aquantum-key-distribution transmitting apparatus 2 and aquantum-key-distribution receiving apparatus 4. The transmittingapparatus 2 can be any mobile device such as a mobile phone, PersonalDigital Assistant (PDA), laptop, digital camera etc; preferably, thetransmitting apparatus is hand-portable and adapted for hand-heldoperation. The receiving apparatus 4 is a base station and can beincluded in any convenient location or equipment such as an ATM,information point, cashier's terminal, etc. For the purpose ofillustrating this embodiment of the present invention, the transmittingapparatus 2 is a mobile phone and the receiving apparatus 4 is a bank'sATM.

The transmitting apparatus 2 includes emitters for three channelsbetween the transmitting apparatus 2 and the receiving apparatus 4. Thefirst channel 6 is a classical communication channel (that is, one notrelying on quantum technology) such as an Infrared Data (IrDA),BLUETOOTH (Trade Mark) or the normal wireless communication channel ofthe mobile phone. The second channel 8 is a quantum channel provided bythe sending of a quantum signal. The third channel 10 is an alignmentchannel for facilitating directional and angular alignment of thetransmitting apparatus 2 and receiving apparatus 4; in some embodiments,the alignment channel is made up of multiple sub-channels.

A quantum signal, in the present context, is a signal capable ofconveying sufficient data to enable a quantum cryptographic transactionwith another entity. Thus, for example, in one embodiment, a source andtransmitter are required which are capable of preparing and transmittingthe quantum state which it is desired to send to a requisite degree ofaccuracy.

A requirement for the successful transmission of the quantum signal inthe quantum channel 8 is that the quantum signal is correctly alignedwith a quantum signal detector of the receiving apparatus 4, bothdirectionally and such that the polarization directions of thetransmitting an receiving apparatus 2, 4 have the same orientation. Thisis achieved using the alignment channel 10. If it is considered that thetransmitting apparatus emits along a z-direction, with the x, y andz-directions all being mutually orthogonal (see FIG. 1), aimingcorrection refers to relative adjustment of the transmitting andreceiving apparatus (or of components of one or both apparatus) so thatthe z axis of the transmitting apparatus substantially intersects thecentre of the lens 54, and orientation correction refers to relativerotation of the transmitting and receiving apparatus (or of componentsof one or both apparatus) about the z axis to align the directions ofpolarization of polarizers of the transmitting and receiving apparatus2, 4.

Referring to FIG. 2 of the accompanying drawings, the transmittingapparatus 2 is shown to comprise a first (classical) channel transceiver12, a second (quantum) channel emitter 14 and a third (alignment)channel emitter 16.

The first (classical) channel emitter 12 is a IrDA transmitter. Thisprovides a data-communication channel for wireless communication betweenthe transmitting apparatus 2 and receiving apparatus 4.

Referring to FIGS. 2 and 3 of the accompanying drawings, the second(quantum) channel emitter 14 comprises an array 18 of light emittingdiodes (LEDs) 20, 22, 24 and 26. Referring now specifically to FIG. 2,in front of each LED 20, 22, 24 and 26 is a respective polarising filter28, 30, 32, 34. Filter 28 polarises the photons emitted from LED 20vertically, filter 30 polarises the photons emitted from LED 22horizontally, filter 32 polarises the photons emitted from LED 24diagonally and filter 34 polarises the photons emitted from LED 26anti-diagonally (the directions of polarisation are stated relative toan intended orientation of the apparatus 2 when in use). Thus, afterpassing through the filters 28, 30, 32, 34, the photons are polarised infour directions, each at 45° to another thus providing two pairs oforthogonal polarisations. The LEDs 20, 22, 24, 26 are narrow frequencyemitters such as those available from Agilent Technologies, Inc. of 395Page Mill Rd, Palo Alto, Calif. 94306, United States e.g., one of theSunpower series, emitting at 590 nm or 615 nm.

A fibre optic light guide 36 is provided to convey the polarised photonsto an attenuation filter 37 and narrow band pass frequency filter 38.The purpose of the attenuation filter 37 is to reduce the number ofphotons emitted and the frequency filter 38 is to restrict the emittedphotons to a narrow frequency range (typically plus or minus 1 nm).Without the attenuation filter 37 in place the number of photons emittedper LED pulse would be of the order of one million. With the filter inplace, the average emission rate is 1 photon per 100 pulses. Importantlythis means that more than one photon is rarely emitted per pulse. Theattenuation filter 37 and frequency filter 38 can be combined in asingle device if preferred. A spatial filter is provided to limit lightleakage outside the channel.

The third (alignment) channel emitter 16 comprises a bright visiblelight laser emitter 40 and a shaped shutter 42 with an aperture in theform of an elongate cross 44 with the centre filled in as illustrated inFIG. 4 of the accompanying drawings. This emits a visible beam of lightin the shape of the elongate cross. Preferably, a central portion thecross-shaped beam is blocked out (see dashed circle 45 in FIG. 4) andthe quantum channel emitter 14 is arranged to transmit its signal alongthe resulting hole in the alignment beam. The cross shape enables theorientation of the transmitting apparatus about the z axis to bedetermined; other beam cross-sectional shapes can be used provided theshape has an asymmetry allowing orientation of the transmittingapparatus to be determined.

The frequency of the LEDs used for the quantum channel is different tothat of the laser emitter 40 used for the alignment channel so as toavoid cross-talk and overload of the quantum channel detectors.

Additionally, the transmitting apparatus 2 comprises a control processor46, a user interface 47, and a memory 48 for storing both data andcontrol programs for controlling operation of the control processor 46to operate the transmitting apparatus 2 as described below.

The receiving apparatus 4 is further explained with reference to FIG. 5of the accompanying drawings. The receiving apparatus 4 comprises aclassical-channel transceiver 50 for receiving signals transmitted inthe classical channel from transmitting apparatus 2, a quantum signalreceiver 52 for receiving the second channel 30 output from transmittingapparatus 2 and a detector 53 for detecting the light signal output fromthe third (alignment) emitter 16.

The quantum signal receiver 53 comprises a lens 54, a yoke 55 forcontrolling the positioning of the lens 54, a quad-detector arrangement85, and a fibre optic light guide for conveying photons received throughthe lens to the quad-detector arrangement 85. The end of the light guide57 nearest the lens 54 is fixed on the optical axis of the lens 55 andis arranged to move with the lens 54 when the latter is moved by theyoke 55. The quad-detector arrangement 85 comprises a beam splitter 56,a first paired-detector unit 80, and a second paired-detector unit 81.The first paired-detector unit 80 comprises a beam splitter 82,polarizers 58, 59, and detectors 60, 61. The second paired-detector unit81 comprises a beam splitter 83, polarizers 62, 63, and detectors 64,65. The polarizers 58, 59 of the first paired-detector unit 80 havetheir directions of polarization orthogonal to each other; similarly,the polarizers 58, 59 of the second paired-detector unit 81 also havetheir directions of polarization orthogonal to each other. Thepolarization directions of the polarizers of the first paired-detectorunit 80 are at 45° to the polarization directions of the polarizers ofthe second paired-detector unit 81. The beam splitters 56, 82 and 83 aredepicted in FIG. 5 as half-silvered mirrors but can be of other formssuch as diffraction gratings.

The detectors 60, 61, 64, 65 are avalanche photo-diodes, such as thoseavailable from Perkin Elmer Optoelectronics of 22001 Dumberry Road,Vaudreuil, Quebec, Canada, J7V 8P7 types C30902E, C30902S, C30921E andC30921S.

Dotted line 86 depicts the paths of photons passing through the lens 54to the detectors 60, 61, 64 and 65 of the quad-detector arrangement 85.

The yoke 55 is a mounting for the lens 54 enabling electromechanicalcontrol of the lens position, using a positioning unit 90 (for example,similar to that used for conventional Compact Disc players). The yoke 55can adjust the position of the lens 54 in a plane generallyperpendicular to the optical path through the lens 54. The quad-detectorarrangement 85 is rotatable about the optical axis of the lens 54 by anorientation unit 91.

The receiving apparatus 4 further comprises a user display 66, a controlprocessor 68 and associated memory 70, configured to operate thereceiving apparatus 4 as described below.

The alignment beam detector 53 comprises a rectangular array 72 of lightdetection elements arranged to detect light of the wavelength emitted bythe laser emitter 40 used in the third emitter 16. This array 72 liessubstantially in the plane of the lens 54 and surrounds the latter withthe central zone 73 of the array 72 being left empty for the lens 54.The output of the array 72 is fed to an alignment control functionalblock 92 which is arranged to analyze this output to determine where andat what orientation the alignment beam is incident on the array 72whereby to provide user feedback via display 66 and control of thepositioning unit 90 and orientation unit 91. The alignment control blockcan be implemented by a program executed by the processor 68 and/or bydedicated circuitry.

The array 72, positioning unit 90, orientation unit 91, alignmentcontrol block 92, and display 66 (to the extent it is used to provideuser feedback) together form an alignment subsystem of the receivingapparatus 4 intended to work in cooperation with the complementaryalignment subsystem of the transmitting apparatus (the emitter 16 of thepresent embodiment) in order to correctly align the transmitting andreceiving apparatus 2, 4.

As will be more fully described below, coarse aiming correction iseffected by user feedback and fine aiming correction by operation of thepositioning unit 90; orientation correction is effected by theorientation unit 91.

A method of operation according to a preferred embodiment of the presentinvention of the apparatus described above will now be described withreference to FIGS. 6 A and B of the accompanying drawings.

The convention is followed that the transmitting side for the quantumsignal is referred to as Alice and the receiving side as Bob. In FIGS.6A and 6B, the appearance of the name of Alice and/or Bob in blockcapitals in relation to a particular step indicates the activeinvolvement of Alice and/or Bob, as the case may be, in that step.

When a user activates the transmitting apparatus 2 in step 100 (FIG. 6A)via the user interface 47, Alice will initiate a dialog with Bob usingthe IrDA channel. Alice tells Bob who she is and Bob responds by tellingAlice who he is. According to the present embodiment, this is done usinga cache of shared secrets possessed by Alice and Bob and eithergenerated by previous interactions between them or downloaded from atrusted source. Typically, the shared secrets will be of the order of100 kbits to 10 Mbits long. The shared secrets=a∥b∥c∥rest_of_secretswhere a, b and c are, for example, each 64 bits (the symbol ∥representing string concatenation). In step 102, Alice transmits (a) XOR(b) to Bob where XOR is the exclusive function. In step 104, Bobsearches through his set of shared secrets looking for a match. Once thematch is found, in step 106 Bob transmits (a) XOR (c) back to Alice. Instep 108, Alice checks that this is the correct response. Both Alice andBob then, in step 110, delete a, b and c from their set of sharedsecrets. i.e. shared secrets=rest_of_secrets.

In step 112, a user activates the alignment channel, typically bydepressing an appropriately marked key on the relevant device. Thiscauses the third emitter 16 to emit a bright visible beam of lightthrough shutter 42 in the manner of a torch. Thus the user sees anelongate cross when the emitted beam strikes a suitable surface.

In step 114, the user uses the cross as a directional guide to aim theoutput towards the receiving apparatus 4.

As the user nears the target receiver (i.e. the lens 54), the alignmentbeam illuminates the array 72 enabling a determination to be made as towhich way the beam should be moved to centre it on the array and thus onthe lens 54. In step 116, the display 66 of the receiving apparatus 4provides a visual indication of the direction in which the user shouldmove the transmitting apparatus 2. This may be in the form of adirectional arrow, a colour showing where the current aim of thetransmitting apparatus 2 lies or any other indicia. The displayadditionally provides in step 118 an “on target” signal when thetransmitting apparatus 2 is correctly aimed at the receiving apparatus4, this signal being, for example, in the form of a displayedword/phrase, a circle around the centre of the target or any othersuitable indicia. An audible signal can additionally/alternatively beprovided.

The beam detector 53 in step 120 uses the asymmetricc shape (elongatecross shape) of the beam emitted as the orientation signal from laseremitter 40 to determine the orientation of the transmitting apparatus 2and cause the orientation unit 91 to rotate the quad-detectorarrangement 85 so as to adjust the orientation of the polarising filters58, 59, 62, 63 such that vertical/horizontal and diagonal/anti-diagonalquantum signals are received appropriately. To minimise the degree ofrotation required of the quad-detector arrangement 85, either of thepaired-detector units 80, 81 can be used as the vertical/horizontaldetector whilst the other unit is used as the diagonal/anti-diagonaldetector.

Slight errors in the directional alignment of the transmitting apparatus2 relative to the receiving apparatus 4, such as those caused by minorhand movements, can be accommodated by the positioning unit 90 adjustingthe position of the lens 54 in step 122. Thus, when the beam detector 53determines that the alignment signal is off-centre, the positioning unit90 is used to adjust the lens position in the plane of the lens 54 tocorrect the alignment of the quantum communication signal.

When alignment is achieved, the quantum signal emitted by the emitter 14will pass through the lens 54 and be guided by optical fibre 57 to thequad-detector arrangement 85, and the polarization directions of thesignal will align with those of the quad-detection arrangement 85.

Once the quantum channel has been established, a quantum key transfercan be made. The transfer of information based on quantum cryptographyis carried out using a variant of the BB84 quantum coding scheme. Thespecific algorithm according to the preferred embodiment will now bedescribed.

Alice and Bob have a predetermined agreement as to the length of a timeslot in which a unit of data will be emitted. To achieve initialsynchronisation, Alice in step 124 (see FIG. 6B) overdrives thealignment emitter 40 to produce a “START” synchronisation signal.Alternatively, the quantum signal channel can be used forsynchronisation.

In step 126, Alice randomly generates a multiplicity of pairs of bits,typically of the order of 10⁸ pairs. Each pair of bits consists of amessage bit and a basis bit, the latter indicating the pair ofpolarization directions to be used for sending the message bit, be itvertical/horizontal or diagonal/anti-diagonal. A horizontally ordiagonally polarised photon indicates a binary 1, while a vertically oranti-diagonally polarised photon indicates a binary 0. The message bitof each pair is thus sent over the quantum signal channel encodedaccording to the pair of polarization directions indicated by the basisbit of the same pair. Randomness in generating the pairs of bits can beachieved by a hardware random number generator such as a quantum-basedarrangement in which a half-silvered mirror is used to pass/deflectphotons to detectors to correspondingly generate a “0”/“1” with a 50:50chance; an alternative form of random number generator can beconstructed based around overdriving a resistor or diode to takeadvantage of the electron noise to trigger a random event.

When receiving the quantum signal from Alice, Bob randomly chooses whichbasis (pair of polarization directions) it will use to detect thequantum signal during each time slot and records the results.

The sending of the message bits of the randomly-generated pairs of bitsis the only communication that need occur using the quantum channel. Theremainder of the algorithm is carried out using the classical channel.

In step 128, Bob informs Alice of the time slots in which a signal wasreceived and the basis (i.e. pair of polarization directions) thereof.

In step 130, Alice sends to Bob confirmation of which of those bases iscorrect. Alice and Bob then use the bits corresponding to the time slotswhere they used the same bases, as the initial new shared secret data.However, there may well be discrepancies (errors) between the versionsof the new shared secret data held by Alice and Bob due, for example, tonoise in the quad detector arrangement 85.

In step 132, error rate checking is carried out by Alice and Bobcomparing their versions of a selected subset of the initial new sharedsecret data. The higher the error rate, the greater the probability isthat the quantum signal has been intercepted. Error rates above about12% are generally unacceptable and, preferably, an upper threshold of 8%is set since above this figure the number of bits available after errorcorrection and privacy amplification is too low.

If the error rate is found to be greater than the 8% threshold, thesession is abandoned and the new shared secret data is discarded (step134).

If the error rate is below the 8% threshold, error correction is thencarried out on the initial new shared secret data (after the latter havebeen reduced by discarding the subsets used for error ratedetermination).

Error correction is effected using a version of the CASCADE algorithm inwhich two basic steps 136, 138 are repeated until a stable condition isreached (typically after six or seven iterations); alternatively, and asindicated by step 140 in FIG. 6B, the number of iterations can be fixed.The two basic steps are:

-   (1) A preliminary step 136 in which Alice and Bob effect the same    random permutation of their respective versions of the new shared    secret data. This is done as follows. Alice and Bob use the same    subset of bits (typically 64 bits) of their new shared secret data    as a seed for a deterministic pseudo random number generator. This    pseudo random number generator is used to permute the data. This way    both Alice and Bob will permute their data in the same way. The    shared secret is then reduced by the subset used as the seed for the    random number generator. This permutation step is designed to do two    things—it uniformly redistributes the bits in error and also make    life difficult for external observers (who do not know how the bits    are being redistributed).    -   The remaining new shared secret data is then treated as if        divided into blocks of a size chosen such that for the measured        error rate each block has, on average, one error.-   (2) An error elimination step 138 in which Alice and Bob process    each block of their respective versions of the shared secret data as    follows. Both Alice and Bob determine the parity of the block and    Bob sends its parity value to Alice. If Alice finds that Bob's    parity value is the same value as Alice has determined for her    block, that block is accepted as error free (although it could have    any even number of errors); if Alice finds that her parity value    differs from Bob's, the block is assumed to have one error (though    it could have any odd number of errors); in this case, a binary    search process is followed to track down the error. This search    process involves the steps of halving the block in error, and    determining which half contains the error by Bob sending Alice the    parity of one of the half blocks which Alice compares with her    parity value for the corresponding half block in her possession; if    the parity values differ, the errored half block is the one being    processed whereas if the parity values are the same, the errored    half block is the one not being processed. The foregoing steps are    then repeated for the errored half block and so on until the errored    bit is identified). The errored bit is then either discarded or Bob    flips the value of his version of the bit.

The above-described error correction process will generally achieve anerror level of 1:10⁶ or better which is sufficient for present purposes.

However, it will be appreciated that the error correction processinvolves the exchange of considerable amounts of parity informationbetween Bob and Alice which is potentially of use to an eavesdropper. Itis also to be noted that although the error-rate-based intercept checkcarried out in step 132 will detect interception of any substantialportion of the quantum signal transmission, an eavesdropper may still beable to successfully intercept a small number of bits of the quantumsignal as there will be a finite (though very small) probability thatmore than one photon is sent during a time slot over the quantum channelthereby leaving open the possibility that an eavesdropper with a beamsplitter can capture one photon while allowing Bob to receive the otherphoton. Accordingly, a privacy amplification step 142 is next performed.In this step both Alice and Bob reduce the size of their respectiveversions of the new shared secret data using a deterministic randomizingpermutation, the reduction in size being dependent on the amount ofparity information exchanged and the level of security required.

A detailed discussion of privacy amplification can be found, forexample, in the paper “Generalized Privacy Amplification”, C. H.Bennett, G. Brassard, C. Crepeau, and U. M. Maurer; IEEE transactions onInformation Theory, IT-41 (6), p1915-1923. In general terms, it can besaid that if the new shared secret x has a length of n bit after errorcorrection, and the eavesdropper has at most k deterministic bits ofinformation about the new shared secret, then if an appropriate class ofhash function h( ) is applied to the secret random data:{0, 1}^(n)→{0, 1}^(n·k−s)where s is a safety parameter 0<s<n−k, the eavesdroppers expectedinformation on h(x) is no more than (2^(−s)/ln 2) bits. Thus varying thevalue of (n−k−s) gives different levels of security for the result ofthe hash of x; in particular, increasing s increases the level ofsecurity.

After the error correction and privacy amplification, Alice and Bob arevery likely to have the same result. However, in step 144 Alice and Bobseek to re-assure themselves that this is the case by exchanging a hashof their new shared secret data; to protect the transmitted hash, it isXORed with bits popped from the store of shared secrets. If the hashesdiffer (checked in step 145), the newly shared data is discarded (step146) together with the bits used from the store of shared secrets.

On the assumption that Alice and Bob have the same new data, they mergethe new data in with the existing shared secret. This merging involvesthe use of a hash function to ensure that the external observer has noknowledge of the final shared secret. Data from this new shared secretis then used to generate a session key (for example, a 128 bit sessionkey) for encrypting the ex change of application data between thetransmitting apparatus and receiving apparatus over the classicalchannel, the data used for creating the session key being discarded fromthe shared secret.

The quantum signal element of the quantum key distribution need onlytake 0.5-1.0 seconds so the user is not required to keep thetransmitting apparatus 2 on target for a long period.

It will be appreciated that many variations are possible to theabove-described embodiment of the invention.

For example, provision can be made for ensuring that the plane of thelens 54 is adjusted to be at least nearly orthogonal to the z axis ofthe quantum signal emitter since although the quantum signal detector 52described about will tolerate some misalignment between the z axis ofthe emitter and the optical axis of the lens 54, if the misalignment istoo great, photons passing through the lens may not be channelled to thequad-detector arrangement. To this end, an element of the array 72 isreplaced with an opaque plate 87 formed with a small aperture 88 behindwhich is an array 89 of light detectors (shown dashed in FIG. 5). Whenthe alignment beam falls on the array 72, the angle between the z axisof the transmitting apparatus 2 and the optical axis of the lens 54 willdetermine which of the detectors of the array 89 will be activated. Thisis illustrated in FIG. 7A in which:

-   -   dotted line 150 illustrates the path of the alignment beam        through the aperture in plate 87 when the plane of the lens is        orthogonal to the z axis of the quantum signal emitter—in this        case, the beam strikes the central detector 151 of the array 89;        and    -   dotted line 155 illustrates an example path of the alignment        beam through the aperture in plate 87 when the plane of the lens        is not orthogonal to the z axis of the quantum signal emitter—in        the illustrated case, the beam strikes a detector 155 of the        array 89.

Depending on which detector of the array 89 is illuminated by thealignment beam, the angle of the lens 54 is adjusted by rotating itabout orthogonal axes lying in the plane of the lens 54 (the yoke 55 andunit 90 being adapted, for example, to perform this task in response tosignals from the control unit 92, this latter being fed with the outputfrom the array 89). The angle of the plate 87 and detector array 89 aresimilarly adjusted (for example, by mechanical linkage with the yoke 55)whereby upon the plane of the lens 54 becoming orthogonal to the z axisof the quantum signal emitter, the alignment beam 155 will strike thecentral detector 151 of the array 89 (see FIG. 7B) causing angularadjustment of the lens 54, plate 87 and array 89 to be discontinued. Theadjustment of the angling of the lens 54 to make it orthogonal to the zaxis of the quantum signal emitter, can be considered as part of theoverall alignment process.

Although in the embodiment described above, a single lens 54 is used, aplurality of independent lenses can be provided either leading to acommon quad-detector arrangement for all such lenses or to a respectivequad-detector arrangement for each lens. In this manner, the operativetarget area is effectively increased and it no longer necessary to mountthe lenses on a yoke to compensate for small alignment errors.

Indications of any suitable sort can be used to guide a user to centrethe quantum signal on the receiving apparatus 4 using the alignmentbeam. For instance, an audible indication can be used with beeps ofincreasing frequency the nearer the receiving apparatus 4 the user aimswith a continuous noise when the signal is centred.

Equally, though it is convenient for the alignment signal to be visual,it need not be.

As an alternative embodiment, the alignment signal can be emitted usingpolarised photons of predetermined polarisation, whereby thepolarisation of the photons is used as the orientation signal by thereceiving apparatus 4. In this embodiment a polarising filter isutilised in front of the alignment signal emitter. The polarising filtermay be rotated through 90° periodically to assist the receivingapparatus 4 in receiving the orientation signal. The receiving apparatus4 is modified by having a corresponding polarising filter in front ofthe orientation signal detector, which detector and polarising filter isrotated until the orientation signal is received, thus determining anorientation of the transmitting apparatus 2 relative to the receivingapparatus 4.

Another simple way of detecting polarization orientation errors is toprovide the mobile device with tilt sensors, the outputs of thesesensors being sent over the classical communications channel to thereceiving apparatus to enable the latter to automatically adjust theorientation of the quad-detector arrangement.

In the illustrated embodiments of the invention a single laser beamemitted from the alignment channel emitter 16 is used for both aimingand orientation alignment. It will be appreciated that rather thanrelying on a single alignment channel signal for all aspects ofalignment, separate alignment signal (forming respective alignmentsub-channels) can be used for the different alignment adjustmentsneeded.

In another variant, depicted in FIG. 8, the above-described yokearrangement 55 is replaced by a tip/tilt mirror 200 the angle of whichis set by a mirror drive unit 201 in dependence on the output of thealignment control 92 such as to compensate for alignment errors andensure that the quantum signal passes via the now static lens 54 to thequad detector arrangement 85. Rather than relying on the alignmentdetector 53 to determine the amount of adjustment to be applied usingthe tip/tilt mirror (as was the case for adjustment of the yoke), inanother embodiment a further detector array 202 is positioned around theaperture of the quad detector 85 to detect quantum signal misalignment(as depicted in FIG. 8 by the dotted rays 204 and 205). The detectorarray 202 is sensitive to a laser beam emitted by the QKD transmittingapparatus that is sent along the path of the quantum signal channelprior to the quantum signal being transmitted (it will be appreciatedthat the adjustment of the tip/tilt mirror therefore takes place beforethe adjustment of the orientation of the quad detector that is effectedfor the purpose of polarization alignment, the quantum signal replacingthe laser beam before the polarization alignment step; it will also beappreciated that the quad detector may need temporary screening toprotect its sensitive detectors from the laser beam).

Although in the described embodiments the quantum signal emitter hasbeen placed in the mobile device and the quantum signal detector in thecomplementary base station apparatus, it would alternatively be possibleto put the quantum signal emitter in the complementary apparatus and thequantum signal detector in the mobile device. For cost reasons, however,mechanical adjustment mechanisms for effecting aiming and orientationalignment are preferably kept in the complementary apparatus andappropriately modified.

It would also be possible for the alignment signals to be emitted by thecomplementary apparatus and detected at the mobile device, the latterthen providing feedback over the classical communication channel to thecomplementary apparatus to enable it to take appropriate alignmentcorrection action, including by way of visual/audible feedback to theuser. Alternatively, visual/audible feedback to the user can be provideddirectly by the mobile device.

In the situation where the quantum signal detector is provided at thecomplementary apparatus and has a significant operative area (forexample, due to the replication of detector elements), it may bepossible to eliminate any fine alignment adjustment action (such aseffected using the yoke arrangement 55 of the FIG. 5 embodiment or thetip/tilt mirror 200 of the FIG. 8 embodiment); however, it is generallypreferred to at least provide the user with audible/visual feedback thatthey are on target.

Whilst it is preferred to automatically correct for polarizationorientation discrepancies between the mobile device and thecomplementary apparatus, it is also possible to arrange for feedback tobe provided to the user to get the user to appropriately rotate themobile device.

In a further variant, where the mobile device is a camera phone, it ispossible to electronically place aiming cross-hairs on the image seenthrough the camera functionality of the device, these cross hairsindicating both where the quantum signal emitter of the device is beingpointed and its polarisation orientation.

Thus, preferred embodiments of the present invention provide anapparatus enabling a possibly unsteady user to correctly line up andorientate a QKD transmitter-receiver pair to enable a quantum keydistribution to take place. The mobile device 2 is portable in that itcan conveniently be carried by a user, and although only effective overa relatively short range (typically 3-5 metres), is usable for quantumkey distribution in typical consumer environments such as a high street,shop, bank etc. In many expected applications, an optional range of lessthan 1 metre will suffice. By providing apparatus enabling freestanding(i.e. no tripods, clamps etc.) to be used in a quantum key distribution,the use of the technique can extend into everyday devices.

1. A device for quantum key distribution, herein QKD, using a quantumsignal of polarized photons, the QKD device being intended for use withcomplimentary QKD apparatus, the device comprising: a QKD subsystemcomprising one of a QKD transmitter and receiver for inter-working witha complimentary QKD receiver or transmitter of said apparatus; and analignment subsystem arranged to wirelessly interact with said apparatusto enable generation of adjustment signals for use in aligning the QKDtransmitter and receiver such that the QKD transmitter is pointing atthe QKD receiver and the polarization axes of the QKD transmitter andreceiver are aligned.
 2. A device according to claim 1, wherein thedevice is suitable for hand-held operation.
 3. A device according toclaim 1, wherein the QKD subsystem comprises a QKD transmitter, and thealignment subsystem comprises an alignment beam transmitter for sendingan alignment beam to said apparatus to enable the apparatus to generatefirst said adjustment signals for aligning the QKD transmitter andreceiver such that the QKD transmitter is pointing at the QKD receiver.4. A device according to claim 3, wherein said alignment beamtransmitter is arranged to condition said alignment beam such that saidapparatus is able to use the beam to generate second said adjustmentsignals for aligning the QKD transmitter and receiver such that thepolarization axes of the QKD transmitter and receiver are aligned.
 5. Adevice according to claim 4, wherein said alignment beam transmitter isarranged condition said alignment beam by giving it a predeterminedcross-sectional shape with an asymmetry allowing orientation of thedevice, and thus the orientation of the QKD transmitter/receiver of theQKD subsystem, to be determined.
 6. A device according to claim 4,wherein said alignment beam transmitter is arranged to condition saidalignment beam by imparting it a predetermined polarisation.
 7. A deviceaccording to claim 5, wherein said alignment beam is a laser beam in thevisible spectrum whereby the image created when the beam strikes asurface enables the user to make appropriate alignment adjustmentsmanually.
 8. A device according to claim 1, wherein the alignmentsubsystem is further arranged such that said adjustment signals areusable to align the optical axes of the QKD transmitter and receiver. 9.A device according to claim 3, in which QKD transmitter is configured toemit photons polarised according to a first orthogonal pair and a secondorthogonal pair, whereby the polarisation of the first orthogonal pairis at 45° to the polarisation of the second orthogonal pair.
 10. Adevice according to claim 1, wherein the QKD subsystem comprises a QKDreceiver, and the alignment subsystem comprises an alignment beamtransmitter for sending an alignment beam to said apparatus to enablethe apparatus to generate first said adjustment signals for aligning theQKD transmitter and receiver such that the QKD transmitter is pointingat the QKD receiver.
 11. A device according to claim 1, wherein thealignment subsystem comprises an alignment beam receiver for detectingan alignment beam sent by said apparatus, the alignment subsystem beingarranged to generate, in dependence on the incidence of the alignmentbeam on the alignment beam receiver, first adjustment signals foraligning the QKD transmitter and receiver such that the QKD transmitteris pointing at the QKD receiver.
 12. A device according to claim 1,wherein the QKD subsystem further comprises a processing unit arrange tocooperate with a corresponding unit of the complementary QKD apparatus,by signals exchanged via a classical communications transceiver of thedevice, in order to effect error correction of random material shared bythe QKD device and apparatus via said quantum signal.
 13. Apparatus forquantum key distribution, herein QKD, using a quantum signal ofpolarized photons, the QKD apparatus being intended for use withcomplimentary QKD device, the apparatus comprising: a QKD subsystemcomprising one of a QKD transmitter and receiver for inter-working witha complimentary QKD receiver or transmitter of said device; and analignment subsystem arranged to wirelessly interact with said device togenerate adjustment signals for use in aligning the QKD transmitter andreceiver such that the QKD transmitter is pointing at the QKD receiverand the polarization axes of the QKD transmitter and receiver arealigned.
 14. Apparatus according to claim 13, wherein the QKD subsystemcomprises a QKD receiver, and the alignment subsystem comprises analignment beam receiver for detecting an alignment beam sent by saiddevice, the alignment subsystem being arranged to generate, independence on the incidence of the alignment beam on the alignment beamreceiver, first said adjustment signals for use in aligning the QKDtransmitter and receiver such that the QKD transmitter is pointing atthe QKD receiver.
 15. Apparatus according to claim 14, wherein thealignment subsystem further comprises audio and/or visual signallingarrangement for generating audio and/or visual alignment feedback to auser of said QKD device in dependence on said first adjustment signals.16. Apparatus according to claim 14, wherein the alignment subsystemfurther comprises an electromechanical alignment unit for adjusting theposition of a collecting lens of the alignment beam receiver independence said first adjustment signals.
 17. Apparatus according toclaim 14, wherein the alignment subsystem further comprises anelectromechanical alignment unit for adjusting the orientation of atip/tilt mirror of the alignment beam receiver in dependence on saidfirst adjustment signals.
 18. Apparatus according to claim 14, whereinsaid alignment subsystem is arranged to detect a conditioning of saidalignment beam indicative of the polarization axes of the QKDtransmitter whereby to generate second said adjustment signals foraligning the QKD transmitter and receiver such that the polarizationaxes of the QKD transmitter and receiver are aligned.
 19. Apparatusaccording to claim 18, wherein the alignment subsystem further comprisesan electro-mechanical polarization-alignment unit for adjusting theorientation of the QKD receiver about its optical axis in dependence onsaid second adjustment signals.
 20. Apparatus according to claim 18,wherein said alignment subsystem is arranged to detect a saidconditioning of said alignment beam in the form of a predeterminedcross-sectional shape with an asymmetry allowing orientation of the QKDtransmitter to be determined.
 21. Apparatus according to claim 18,wherein said alignment subsystem is arranged to detect a saidconditioning of said alignment beam in the form of a predeterminedpolarisation.
 22. Apparatus according to claim 13, wherein the alignmentsubsystem is further arranged such that said adjustment signals areusable to align the optical axes of the QKD transmitter and receiver.23. Apparatus according to claim 13, in which the QKD receiver comprisesa detector for horizontally polarized photons, a detector for verticallypolarized photons, a detector for diagonally polarized photons and adetector for anti-diagonally polarized photons.
 24. Apparatus accordingto claim 13, wherein the QKD subsystem further comprises a processingunit arrange to cooperate with a corresponding unit of the complementaryQKD device, by signals exchanged via a classical communicationstransceiver of the apparatus, in order to effect error correction ofrandom material shared by the QKD device and apparatus via said quantumsignal.